What sort of filter you purchase is in large part decided by the contamination you wish to remove and what services you expect to be available to you.
Home/RV RO systems are more affordable all the time and are brilliant but require a external power source to run - Your vehicle/boat is adequate. If you've got a large quantity of suspended solids - run it through a sand filter or use a flocculent to clear it first.
I've got a carbon filter here I made up for post-treating alcohol that works brilliantly and wasn't much more than $50 in materials to build - and is easy enough to service. I still boil the living crap outta water poured through this after - just to be sure.
If you're going to be on foot - those "straw" types of carbon filter will function. To prolong their life you can pre-treat the water by letting it settle, using a sand filter or stocking or what not to get rid of the big lumps that will clog the pores in the carbon very quickly.